This invention relates to an ambipolar electrophotgraphic plate and to the method for manufacture thereof.
In the art of xerography, a xerographic plate containing a photoconductive insulating layer is imaged by first uniformly electrostatically charging its surface. The plate is then exposed to a pattern of activating electromagnetic radiation such as light, which selectively dissipates the charge in the illuminated areas of the photoconductive insulator while leaving behind a latent electrostatic image in the non-illuminated areas. This latent electrostatic image may then be developed to form a visible image by depositing finely-divided electroscopic marking particles on the surface of the photoconductive insulating layer.
The use of vitreous or amorphous selenium remains the most widely used photoreceptor in commercial reusable xerography. A amorphous selenium is capable of holding and retaining an electrostatic charge for relatively long periods of time when not exposed to light, and is relatively sensitive to light as compared to most other photoconductive materials.
Amorphous selenium conducts both electrons and holes, but the mobility of the holes is approximately ten times greater than that for electrons. Thus, it can be stated that amorphous selenium while possessing a long range for holes has a very short range for electrons. The effect of this characteristic on the xerographic utility of selenium can best be understood by examining the basic process steps of xerography. As stated above, an amorphous selenium photoconductive layer is first sensitized by placing a uniform electrostatic charge on the surface of the photoconductive insulating material. This uniform charge creates a relatively strong field across the selenium (generally relative to a conductive backing). The amorphous selenium is then exposed to radiation to which it is sensitive, usually in the blue-green portion of the visible spectrum. The absorption of activating radiation acts to create hole-electron pairs in the selenium at the point of absorption of the impinging radiation. If the sensitizing charge on the surface of the selenium is negative, positive charge created by the radiation migrates to the surface to neutralize existing negative charges while the photogenerated negative charges are repelled by the remaining sensitizing charge to migrate through the selenium toward the conductive backing. When the sensitizing charge on the surface of the selenium is positive, the reverse is true. Electrons created by the radiation migrate to the surface to neutralize positive charges and the photogenerated holes or positive charge carriers are repelled to migrate through the selenium to the conductive backing. Inasmuch as selenium has a very short range for electrons, when used with negative charging, the result is that a large number of electrons are trapped in the bulk of the selenium layer, thereby rendering the plate unfit for further use in xerography until the trapped charges are freed. In that selenium has a long range for holes, when used with positive sensitizing charges, trapping is reduced to a sufficiently small degree so as not to interfere with the utility of the material for xerographic processes. It therefore has become the usual practice in xerography, when using amorphous selenium, to employ positive polarity sensitizing charges at its surface.
There are many applications in electrophotography where it is desirable that amorphous selenium or other photoconductive layers have long range for both electrons and holes so that they can be used for both positive and negative charging characteristics, as for use in obtaining a reversal of the image, as in reversal microfilm printing and in laser printers.
As for example, it is desirable that amorphous selenium based photoreceptors have long range for both polarities of charge carriers when they are used in obtaining a reversal of the image to be reproduced in the normal xerographic process. In this case, if the normal xerographic plate is charged negatively and then the steps of the xerographic process are carried through, including development with carriers and toners as described for the normal xerographic process, there is obtained a negative or reversed image of the copy being reproduced. Thus, if the plate has a long range for both polarities of charge carriers it is possible merely by altering the polarity of the sensitizing charge to obtain either a positive or reversal reproduction of the subject matter being reproduced.
U.S. Pat. No. 3,077,386 to Blakney et al. describes one technique for treating selenium whereby the material acquires the property of having a long range for both polarities of charge carriers. This technique involves doping the selenium with a small amount of metal such as chromium, nickel, iron, zinc, calcium, titanium, or other similar material.
Another technique is described in U.S. Pat. No. 3,685,989 wherein use is made of amorphous selenium or an arsenic alloy of selenium which contains a small amount of sodium, lithium, potassium, rubidium, cesium or mixtures of the above wherein such alloying elements are present in an amount within the range of 5-5000 parts per million by weight.
Another technique is described in U.S. Pat. No. 3,712,810 wherein use is made of a thin layer of thallium doped amorphous selenium or thallium doped selenium-arsenic contained on the substrate, and a layer of amorphous selenium or selenium-arsenic overlaying the thallium doped layer.